Technical Abstract:
Stomatal responses to atmospheric change have been well documented through a range of laboratory and field-based experiments. Increases in atmospheric concentrations of both CO2 ([CO2]) and O3 ([O3]) have been shown to decrease stomatal conductance for a wide range of species under numerous conditions, although the mechanisms for response between [CO2] and [O3] vary. Less well understood, however, is the extent to which leaf level responses translate to changes in ecosystem evapotranspiration (ET). Since ET can be influenced both by physiology and physical characteristic of the plant canopy, a response at the leaf level may not be translated to an observable response at the ecosystem level. To test this, maize (Zea mays) was grown in elevated [CO2] and soybean (Glycine max) in an elevated [CO2] and elevated [O3] factorial experiment under fully open-air conditions at the SoyFACE research facility in Urbana, IL. Ecosystem ET was measured from canopy closure to senescence using a residual energy balance approach. Latent heat of evaporation (ET) from crop stands were estimated from continuously measured net radiation, sensible and soil heat fluxes. Elevated [CO2] caused ET to decrease between 9 and 16% and elevated [O3] caused decreases of 7 to 27% for soybean, depending on year. The magnitude of the ET response for soybean mimicked the leaf-level stomatal conductance data collected at SoyFACE for the three years in which the measurements overlapped. Elevated [CO2] also reduced ET for maize by almost 40% during the one year in which it was measured. The decrease in ET raised the water use efficiencies for all treatments which carries important implications for regional climatology. This is particularly true of continental interiors, such as the Midwest Corn Belt, where a large portion of atmospheric humidity during the growing season is provided by transpiration.